1. Field of the Invention
This invention relates to heating systems and more particularly it concerns novel arrangements for preheating solid aggregate material, such as scrap metal, to drive off volatiles from the material before the material is melted.
2. Description of the Prior Art
U.S. Pat. No. 3,383,099 shows a preheating system suitable for heating scrap metal prior to charging the metal into a melting furnace. In that system a charge of scrap metal to be preheated is held in a bucket and hot gases from a group of gas or oil burners are driven through the charge. This arrangement has been found to provide very rapid and efficient preheating of metals. In addition, the burners of such preheaters can operate at very high temperatures for long periods of time without need for replacement. Also the combustible volatiles which are driven off during preheating can be burned in the process and, to a certain extent their heat of combustion can be recovered. Some inefficiency occurs in these systems, however, because of their need to heat large amounts of combustion air which is later exhausted as waste heat.
It has also been proposed to employ electrical heating systems of the type shown and described in U.S. patent application Ser. No. 701,013 filed June 30, 1976, and assigned to the assignee of the present invention, for preheating scrap metal. These electrical preheating systems utilize heating elements which are raised to high temperature by passing large electrical currents through them; and the heat developed by these elements is radiated onto the scrap metal. These electrical heating systems are efficient in that they do not require the heating of combustion air. However they do not have the capability to make effective use of the heat of combustion of the combustible volatiles driven off from the scrap metal.